1. Field of the Invention
The present invention relates to an electrostatic recording apparatus.
2. Discussion of Background
There are, as a system for developing an electrostatic latent image formed on a photosensitive drum of an electrophotograph or an electrostatic image drum of an electrostatic recording apparatus, a two-component development system using two-component development agent having magnetic carrier and powder toner, a magnetic one-component development system using magnetic toner, a nonmagnetic one-component development system using only nonmagnetic powder toner and a liquid development system.
The liquid development system is adapted for a color recording having fine toner and high picture quality. In addition, the liquid development system can develop a low-voltage electrostatic latent image at 200 V or lower. However, the liquid development system needs high insulated kerosene used for jet fuel as a carrier for imparting charge to the toner, the high insulating kerosene is used only for special purposes such as a plotter due to danger and offensive odor, and is not used for a general purpose printer or copying machine in an office.
The magnetic one-component development system can develop low electrostatic contrast similar to the liquid development system, and additionally conveys toner by a magnetic element in the toner, and hence has a simple structure for a development device. However, since the magnetic one-component development system uses intensity of polarization of toner generated by an electrostatic image at the time of developing, it is not adapted for a system for reverse development of a region having no charge like a laser printer. Since the magnetic one-component development system has a conductive magnetic element in the toner, a toner image formed on a recording medium cannot be electrostatically transferred to a normal sheet. Further, since the magnetic one-component development system has an opaque black magnetic element, it is not adapted for coloring.
The nonmagnetic one-component development system is a system improved as compared with the above-described development systems at the stage of a practical component development system a contact development system in which a surface of a development roller is coated with toner and contacted with a recording medium, and a non-contact development system in which an image drum is not contacted with a development roller.
The contact development system in which the development roller is contacted with a photosensitive drum causes an electrostatic latent image on a recording medium to be varied according to a frictional discharging between the development roller and the recording medium, a white background having a low electrostatic contrast is easily fogged, and image noise is readily generated. Therefore, the contact development system removes the fog by a high development bias voltage being 400 V or higher, removes image noise by forming a high electrostatic contrast and developing the image up to saturated density. As a result, the contact development system is adapted for binary recording by gradation recording.
As described above, the contact development system necessitates to form a high contrast electrostatic latent image on the image drum, and an orbit of charge is bent due to optical carriers in the image drum generated by a laser light or the like, when a high electrostatic latent image is formed. Thus, an electrostatic pixel enlarges, and hence picture quality is lowered. Further, a large mechanical load is applied in the case of contact of the development roller with the image drum to generate a feeding irregularity of the image drum, thereby causing an irregularity in image density to occur.
Since the nonmagnetic one-component development for developing an image by flying toner between a development roller and a recording medium flies the toner with higher electrostatic power than physical adherence generated between the toner and the development roller, a high DC bias voltage of about 500 V and a high AC bias voltage of 1 kVpp or more are superposed and applied to the development roller.
Since the non-contact development system has no frictional charge between the development roller and the image drum and develops the image by reciprocatingly flying toner between the development roller and the image drum by applying the DC bias voltage and the AC bias voltage, it is preferable without problem of the contact development system.
Since the non-contact development system can develop low electrostatic images at 200 V or lower, an interference between the electrostatic latent image and optical carrier in the image drum generated by irradiating it with a light can be reduced to prevent an extension of the electrostatic pixel, thereby high picture quality can be achieved.
Since a high voltage is applied between the development roller and the recording medium in the non-contact development system in which the DC bias voltage and the AC bias voltage are added and applied to the development roller, a spark discharge is generated between the both at a highland where low discharge is easily generated at a dried low atmospheric pressure not to be able to develop the image.
In the one component development system transferring this multilayer toner, moisture among toner is reduced and no moisture adherence force among toner effects. As a result, the critical bias voltage of flying toner is varied, and picture quality varied and unstable developing by toner scattering occurred.
The two-component development system employs a powder development system for developing an electrostatic latent image of 400 V or higher by mixing magnetic carrier and insulation toner and applying a charge to toner by means of frictional charging of both. The two component development system can obtain stable picture quality and is most generally used for a copying machine, a laser printer, [and further] a digital color printer and the like. However, the two-component development system needs replacement of carrier for applying charge to the toner at several thousand numbers of recording sheets and additionally has a development device of a large size for mixing the carrier and the toner.
A conventional electrostatic development apparatus will be explained with reference to FIGS. 1 and 2.
FIG. 1 is an arrangement view of a non-contact one-component development device, a recording drum and a bias power source of a conventional electrostatic developing apparatus.
FIG. 2 is a view showing the state of flying toner from a development roller to a recording drum.
The toner 11 in the development device 10 is applied by charge by means of friction with charging blade 12, and several charged toner layers are formed on the development roller 15 on a metal surface so roughed in the degree of particle size of the toner 11. Since a distance between the development roller 15 and the recording drum 20 affects an influence to image density, it is normally held accurately in the degree of 200 .mu.m.
The development roller 15 and the recording drum 20 are respectively moved at an equal speed in directions of arrows D1 and D2. The development roller 15 is applied by a DC bias and an AC bias from a DC bias source 31 and an AC bias source 35, respectively. The fogging toner of a white area of a surface potential of -600 V on the recording drum 20 given by precharging is reversely flown by the DC bias voltage of -400 V and the AC bias voltage of 1.5 kVpp and a frequency of 2 kHz thereby [to] preventing fogging. The toner 11 flown to the recording medium in which surface charge of an image area is reduced remains thereat and is developed.
FIG. 3 is a view showing characteristics of the nonmagnetic one-component development system of the case where the AC bias voltage is varied, where an abscissa axis is an electrostatic contrast, and a value in which a DC bias voltage is subtracted from a potential of the electrostatic latent image, and an ordinate axis is an image density. A curve CO is developing characteristics of the state that the AC bias voltage is not applied, and a high electrostatic contrast of 600 V or higher is required for the development. A curve CG is developing characteristics of the case where the AC bias voltage of 1.5 kVpp is applied, and the electrostatic contrast necessary for developing is 200 V or lower, thereby exhibiting high gamma characteristics.
The gamma characteristics which exhibit gradation characteristics can be controlled by altering the AC bias voltage. However the AC bias voltage is varied, a critical voltage Vc of the development is altered, and alteration of the gradation characteristics of the image by controlling the gamma characteristics is not achieved. The nonmagnetic one-component development system needs a high accuracy to set a position between the development device and the recording drum.
Ozone is generated in the case of charging by charge in an electrostatic developing apparatus in which the above-described electrostatic image is formed. Since the generation of the ozone causes an office environment to be deteriorated, a charger for improving the office environment by reducing an ozone generation amount for a printer has been recently developed. Thus, a roller Charging has been vigorously developed. Since a low surface potential is advantageous to obtain a uniform image by this roller charging, a request for a low potential development device for developing an electrostatic image of low voltage of 200 V is taking place.
As described above, the toner 11 is reciprocated between the development roller 15 and the recording drum 20 by the AC bias voltage, adhered to an image area in which charge on the recording drum 20 is erased, and developed. The toner 11 on the development roller 15 is precluded by the DC bias, which has a function of flying the toner 11 in a direction of the recording drum 20.
FIG. 4 is a graph showing a calculated result of developing characteristics when physical adhering strength between the toner 11 and the development roller 15 is varied. In this case, the physical adhering strength is varied from 1.times.10.sup.-8 (N) to 1.times.10.sup.-7 (N). In FIG. 4, an abscissa axis is an electrostatic contrast, and an ordinate axis is image density.
The physical adhering strength is van der Waals force and an adhering strength by the moisture influenced by environmental conditions. An electric field necessary to fly the toner 11 is large at a curve C1 changing an adhering strength by moisture and increasing a physical adhering strength, and a high electrostatic contrast at 400 V or larger being necessary to a saturated concentration and flying start voltage of toner 11 at 100 V or larger are necessary. When the physical adhering strength due to moisture at the time of high moisture exceeds 1.times.10.sup.-6 (N), it is impossible to fly the toner 11 by the electrostatic force. A curve C2 shows the case that the physical adhering strength is low of 1.times.10.sup.-8 (N). The physical adhering strength is reduced as indicated by the curve C2 at a highland where the atmospheric pressure is low and dry, and a spark discharge is simultaneously generated between the development roller 15 by the reduction of the atmospheric pressure and the recording medium to cause the development to be impossible. Thus, when the physical adhering strength is reduced by drying, the toner 11 is scattered by a repelling force of the multi-layer toner 11 on the development roller 15, thereby causing a decrease in picture quality and contamination due to toner 11 of the electrostatic developing apparatus.
On the other hand, the contact development system (the electrostatic developing apparatus in which this system is employed is called "contact one-component development device") causes an electrostatic latent image on the recording medium to be varied due to frictional charging between the development roller 15 and the recording medium, a white area of low electrostatic contrast is easily fogged, and picture noise is readily generated. Therefore, a high developing bias voltage is applied to remove the background fog, and a high electrostatic contrast is formed to remove picture noise to develop to saturation density. As a result, this development system is adapted for binary recording by gradation recording.
Another improved contact one-component development device will be explained with reference to FIG. 6. FIG. 6 shows an arrangement of a non-contact one-component development device, a recording drum and a bias power source of a conventional electrostatic developing apparatus. The development roller has a dielectric element 23 in which an infinitesimal area is isolated on the surface of a conductive development roller 15.
A polarization charge is generated on the isolated dielectric element layer by the conductive surface potential given by charge by frictional charging with the toner 11 or the charging blade. As shown in FIG. 6, a strong electric field in the direction of an array shown in FIG. 5 is generated between the charged dielectric element 23 and a peripheral conductive development roller 15, a large quantity of the toner 11 is fixed to the periphery of the dielectric element 23, conveyed, and developed at a high speed.
The solidable ion generator utilizing discharge is used as a charger for uniformly charging an insulating recording medium to be used without noise for a recorder utilizing electrostatic recording of electrophotograph or the like (called "an ion generator" when an ion generator itself is designated). The necessary improvement of a solid ion generator will be explained.
FIG. 7 is a view showing an example of an electrophotographic process of an ion generator 50.
A charging device 50 constituted by the solid ion generator is used to uniformly charge a photosensitive layer 21.
FIG. 8 is a view showing a structure of a conventional solid ion generator 50. A dielectric electrode 52 of a first electrode and an ion generating electrode 53 of a second electrode are provided at both sides of a dielectric layer 51 (which may also be called a "dielectric").
An AC voltage 70 having a peak value of 2.5 to 3.0 kVpp is applied between the dielectric electrode 52 and the ion generating electrode 53, discharging is generated in the ion generating slit 54, and ions of both positive and negative polarities are generated.
Ions of single polarity are moved from the ion generator 50 to the photosensitive drum by applying a bias voltage between the ion generating electrode 53 and the photosensitive drum of an object to be charged, thereby charging the photosensitive layer 21.
As one of the problems of the solid ion generator capable of uniformly charging to improve the picture quality, the surface of the dielectric layer 51 is degraded by formation generated by discharging might be gradually weakened to reduce charging capacity. As another of the problems, when the surface of the dielectric 51 is degraded, discharging does not immediately occur even by applying an AC voltage, and it takes a time until a charging current is saturated. If the surface of the dielectric layer 51 is degraded to weaken the discharge, when a temperature of the ion generator 50 itself is raised or a peak value of the AC voltage is raised, the discharging is again strengthened to the necessary degree, but the degradation of the surface of the dielectric 51 is advanced, and the discharge is again weakened.
As described above, the ion generator utilizing discharging generates ozone of detrimental substance by discharging.
In a copying machine or a printer utilizing an electrophotographic process, an ozone filter is disposed in a discharge hole from the recorder to reduce ozone density to be discharged out of the apparatus to a certain reference or less. However, it is necessary to periodically replace the ozone filter at a predetermined period. In the case of a copying machine to be used in an office, a maintenance man periodically replaces it and no problem occurs, but in a printer in which a user must conduct maintenance, replacement is ignored, and it frequently discharges ozone of reference or more.
A method for charging by bringing a roller to be applied by the voltage still has a problem that stability and charging irregularity occur even if ozone amount to be generated is small.
As described above, applicant of the present invention who has discussed improvements in picture quality of an electrophotograph or the like has discovered the fact that, when an electrostatic latent image of 200 V or higher is formed on a photosensitive drum, optical carriers in the photosensitive drum generated by irradiating with a light interferes with the electrostatic latent image and an electrostatic pixel is extended (in an ion deposition recording for forming directly the electrostatic latent image on the insulating recording medium by controlling ions at each pixel, since an interference between the electrostatic latent image and an ion flow occurs, and extension of the pixel occurs, the development device same as above is required). When the electrostatic latent image is set to a low electrostatic contrast of 200 V or lower, the interference is reduced, and a high picture quality is formed. As described above, there are as a system for developing a low electrostatic contrast a liquid development system and a magnetic one-component development system, but the above-described drawbacks exist. Therefore, the applicant has aimed at a ion generator capable of uniformly charging without public pollution, for example, ozone or the like, and non-contact non-magnetic one-component development system which can execute a color development. With the development device, a development of an electrostatic developing apparatus in which a spark discharge does not occur by a low physical adhering strength between toner and a development roller is stabilized to stabilize environment of developing characteristics, a low electrostatic contrast of 200 V or lower allows to develop the image, an extension of the pixel is eliminated to obtain a preferable image without rough even on a low density area is a subject of the present invention. With the ion generator, a development of an electrostatic development apparatus which reduces ozone and has a long-life is a subject of the present invention.